By Dillon Lim - Medicine Student @ Brasenose College, Oxford
The skin is composed of three major layers: the epidermis, a layered epithelium, followed by the dermis and hypodermis, layers of denser and looser connective tissue. These different tissues give different layers of the skin their properties and are adapted for their functions in protection, thermoregulation, and as a sensory organ.
The epidermis itself can be divided up into five layers. The stratum basale is the deepest layer, home to the stem cells that proliferate to renew the whole epithelium once every 25-30 days. This is important for the skin’s protective function; skin at the apical (outer) layer sloughs off at the rate of 1g/day. It also contains Merkel’s cells (touch sensors), Langerhans cells (a type of immune cell specific to the skin) and melanocytes. Melanocytes produce packages of the pigment melanin which can then be sent to other cells nearby. UV radiation stimulates increased melanocyte activity as a means to protect DNA against these ionising wavelengths.
Mutations in tyrosinase, the main enzyme used in melanin production, cause oculocutaneous albinism. Individuals with this condition are much more likely to develop skin cancers as a result; this difference is particularly marked in African populations, where albino populations appear to have a 1000x greater incidence of this cancer as opposed to non-albino populations.
In the next levels, cells start to increase the production of keratin, lipids, and proteins rich in an amino acid called cysteine. The lipids are later released to give the skin its waterproof property, which cysteine-rich proteins cross-link to form some stability in the upper layers, along with keratin. The cells at the upper layers of the skin are all dead, with just these biochemical elements holding them in support.
The epidermis is held together by various cell-cell protein junctions which act to hold cells together. Perhaps not surprisingly, mutations affecting these junctions can cause the skin to peel and blister. Pemphigus and pemphigoid are two autoimmune conditions where two types of cell-cell junctions (desmosomes and hemidesmosomes) are attacked by the immune system. The epidermolyses bullosae (EB) are another class of diseases characterised by blistering. Blistering stops the skin being able to act as a protective barrier; EB patients often have to contend with severe skin infections.
The dermis is made of dense irregular connective tissue, for multidirectional strength. It is composed of many types of collagen fibres, which provide both strength and elasticity. The blood supply here dilates or constricts as part of thermoregulation, or the control of body temperature. The dermis is also home to the pilosebaceous appendages. These consist of a hair follicle and shaft, an associated muscle that helps the hair stand (the arrector pili) and sebaceous gland, which secretes sebum. Sebum has waterproofing and antimicrobial properties. Near hair follicles we will also see sweat glands, of which there are two types. The eccrine glands appear nearly all over the body and produce a more watery sweat, while the apocrine glands, found in the armpits and genitalia, produce a more viscous sweat. The difference between the two secretions and the distribution of these two glands is of course why sweaty armpits smell, while a sweaty hand for instance doesn’t - at least in the same way! The dermis also contains free nerve endings for the sensing of heat, pain, and touch.
Below this is the hypodermis, comprised of loose connective tissue. It also contains fatty adipose tissue. Again, there are two distinct types, which have different appearances under the microscope and different uses. White adipose tissue is used for insulation and energy storage, and the fat in inside the cells is held in one huge “droplet”. Brown adipose tissue is burnt for heat, due to the expression of the special protein UCP-1 that allows the energy produced in oxidative phosphorylation to be released, instead of going into ATP production; here, the fat is stored in many smaller droplets.
Further reading:
Structure and function of the skin, hair and nails. https://www.sciencedirect.com/science/article/pii/S1357303917300695
The Structure and Function of Skin. https://www.academia.edu/download/38073877/1The_Structure_and_Function_of_Skin.pdf
Epidermolysis Bullosa. https://rarediseases.org/rare-diseases/epidermolysis-bullosa/